Implantable medical devices for producing a therapeutic result in a patient are well known. Examples of such implantable medical devices include implantable drug infusion pumps, implantable neurostimulators, implantable cardioverters, implantable cardiac pacemakers, implantable defibrillators and cochlear implants. Of course, it is recognized that other implantable medical devices are envisioned which utilize energy delivered or transferred from an external device.
A common element in all of these implantable medical devices is the need for electrical power in the implanted medical device. The implanted medical device requires electrical power to perform its therapeutic function whether it is driving an electrical infusion pump, providing an electrical neurostimulation pulse or providing an electrical cardiac stimulation pulse. This electrical power is derived from a power source.
Typically, a power source for an implantable medical device can take one of two forms. The first form utilizes an external power source that transcutaneously delivers energy via wires or radio frequency energy. Having electrical wires which perforate the skin is disadvantageous due, in part, to the risk of infection. Further, continuously coupling patients to an external power for therapy is, at least, a large inconvenience. The second form utilizes single cell batteries as the source of energy of the implantable medical device. This can be effective for low power applications, such as pacing devices. However, such single cell batteries usually do not supply the lasting power required to perform new therapies in newer implantable medical devices. In some cases, such as an implantable artificial heart, a single cell battery might last the patient only a few hours. In other, less extreme cases, a single cell unit might expel all or nearly all of its energy in less than a year. This is not desirable due to the need to explant and re-implant the implantable medical device or a portion of the device. One solution is for electrical power to be transcutaneously transferred through the use of inductive coupling. Such electrical power or energy can optionally be stored in a rechargeable battery. In this form, an internal power source, such as a battery, can be used for direct electrical power to the implanted medical device. When the battery has expended, or nearly expended, its capacity, the battery can be recharged transcutaneously, via inductive coupling from an external power source temporarily positioned on the surface of the skin.
Several systems and methods have been used for transcutaneously inductively recharging a rechargeable used in an implantable medical device. Some examples of systems and method used for transcutaneously inductively charging or recharging an implantable medical device include U.S. Pat. No. 5,411,537, Munshi et al, Rechargeable Biomedical Battery Powered Devices With Recharging and Control System Therefor, (Intermedics, Inc.); U.S. Pat. No. 5,690,693, Wang et al, Transcutaneous Energy Transmission Circuit For Implantable Medical Device, (Sulzer Intermedics Inc.); U.S. Pat. No. 5,733,313, Barreras, Sr., FR Coupled Implantable Medical Device With Rechargeable Back-Up Power Source, (Exonix Corporation); U.S. Pat. No. 6,308,101, Faltys et al, Fully Implantable Cochlear Implant System, (Advanced Bionics Corporation); U.S. Pat. No. 6,324,430, Zarinetchi et al, Magnetic Shield For Primary Coil of Transcutaneous Energy Transfer Device, (Abiomed, Inc.); U.S. Pat. No. 6,516,227, Meadows et al, Rechargeable Spinal Cord Stimulator System, (Advanced Bionics Corporation); U.S. Pat. No. 6,505,077, Kast et al, Implantable Medical Device With External Recharging Coil Electrical Connection, (Medtronic, Inc.); European Patent Application 1,048,324, Schallhorn, Medical Li+ Rechargeable Powered Implantable Stimulator, (Medtronic, Inc.); PCT Patent Application No. WO 01/83029 A1, Torgerson et al, Battery Recharge Management For an Implantable Medical Device, (Medtronic, Inc.); and PCT Patent Application No. WO 01/97908 A2, Jimenez et al, An Implantable Medical Device With Recharging Coil Magnetic Shield, (Medtronic, Inc.).
U.S. Patent Application Publication US 2005/0075700A1 (U.S. patent application Ser. No. 10/837,506, Schommer et al, External Power Source For An Implantable Medical Device Having An Adjustable Magnetic core and System and Method Related Therefore, filed Apr. 30, 2004), discloses an external power source, and system and method using such external power source, for an implantable medical device having therapeutic componentry and a secondary coil operatively coupled to the therapeutic componentry. A primary coil is capable of inductively energizing the secondary coil when externally placed in proximity of the secondary coil. A repositionable magnetic core associated with the primary coil is capable of being repositioned by a user of the external power source. An indicator is capable of providing the user with information relative to coupling between the primary coil and the secondary coil as a function of repositioning of the repositionable magnetic core.
Transcutaneous energy transfer through the use of inductive coupling involves the placement of two coils positioned in close proximity to each other on opposite sides of the cutaneous boundary. The internal coil, or secondary coil, is part of or otherwise electrically associated with the implanted medical device. The external coil, or primary coil, is associated with the external power source or external charger, or recharger. The primary coil is driven with an alternating current. A current is induced in the secondary coil through inductive coupling. This current can then be used to power the implanted medical device or to charge, or recharge, an internal power source, or a combination of the two.
For implanted medical devices, the efficiency at which energy is transcutaneously transferred is crucial. First, the inductive coupling, while inductively inducing a current in the secondary coil, also has a tendency to heat surrounding components and tissue. The amount of heating of surrounding tissue, if excessive, can be deleterious. Since heating of surrounding tissue is limited, so also is the amount of energy transfer which can be accomplished per unit time. The higher the efficiency of energy transfer, the more energy can be transferred while at the same time limiting the heating of surrounding components and tissue. Second, it is desirable to limit the amount of time required to achieve a desired charge, or recharge, of an internal power source. While charging, or recharging, is occurring the patient necessarily has an external encumbrance attached to their body. This attachment may impair the patient's mobility and limit the patient's comfort. The higher the efficiency of the energy transfer system, the faster the desired charging, or recharging, can be accomplished limiting the inconvenience to the patient. Third, amount of charging, or recharging, can be limited by the amount of time required for charging, or recharging. Since the patient is typically inconvenienced during such charging, or recharging, there is a practical limit on the amount of time during which charging, or recharging, should occur. Hence, the size of the internal power source can be effectively limited by the amount of energy which can be transferred within the amount of charging time. The higher the efficiency of the energy transfer system, the greater amount of energy which can be transferred and, hence, the greater the practical size of the internal power source. This allows the use of implantable medical devices having higher power use requirements and providing greater therapeutic advantage to the patient and/or extends the time between charging effectively increasing patient comfort.
The efficiency of transcutaneous inductive energy transfer is directly related to the accuracy of positioning of the external, primary coil, to the internal, secondary coil. The two coils should be as close to each other as possible. Of course, since the position of the secondary coil is fixed following implantation, the closer that the primary coil can be positioned to the skin surface the better. The two coils should also be laterally aligned as close as possible. This alignment is typically accomplished by the patient by the attachment of the external power source/charger at the commencement of the charging process or when otherwise transferring power. It is often cumbersome and difficult for the patient, who typically is not a medical professional, to most accurately position the primary coil in the proper location. The lateral alignment is typically done tactilely by the patient. A typical implanted medical device is implanted close enough to the skin that the skin of the patient has a small protuberance at the site of implantation. This can be felt by the patient and can be used as a guide to position the external coil. However, this problem can be exacerbated because the lateral position of the secondary coil is not always laterally centered with the external protuberance providing the patient with tactile lateral location information.
Even if the primary coil is properly placed at the initiation of energy transfer or of the charging process, energy transfer and/or charging can continue over a signification period of time. During this time, it is usually impracticable for the patient to remain absolutely immobile. Charging can typically occur over several, perhaps many, hours. It is desirable for the patient to be able to continue with as many normal activities as possible. For example, since charging often is accomplished at night, it is desirable that the primary coil not move during normal sleep activities of the patient. As the patient may move during energy transfer or during charging, motions and activities of the patient may cause the primary coil to move with respect to the secondary coil. If this should happen, the efficiency of energy transfer is not optimum which limits the rate at which energy can be transferred and resulting in an increase in charging time, if the system utilizes charging, or a decrease in the amount of energy available to the implanted medical device, if direct energy transfer is utilized.
It also can be important to secure the primary coil in the proper location once the proper has been located by the patient.
Prior art implantable medical devices, external power sources, systems and methods have not always provided the best possible benefit leading to efficiency of energy transfer and patient comfort.